Group III nitride semiconductors have a direct transition band structure exhibiting a bandgap energy corresponding to visible to ultraviolet light, and enable light emission of high efficiency. Therefore, Group III nitride semiconductors have been employed in LED and LD products. Meanwhile, at the hetero-junction interface between aluminum gallium nitride (AlGaN) and gallium nitride (GaN), a two-dimensional electron layer is generated due to the piezoelectric effect, which is a characteristic feature of a Group III nitride semiconductor. Therefore, even when employed in an electronic device, a Group III nitride semiconductor has a potential to exhibit characteristics which are not exhibited by a conventional Group III-V compound semiconductor.
However, a Group III nitride semiconductor single crystal is difficult to grow because nitrogen exhibits a dissociation pressure as high as 2,000 atm at a temperature at which the single crystal is grown. Therefore, unlike the case of a non-nitride Group III-V (except for a nitride) compound semiconductor, at present, difficulty is encountered in employing a Group III nitride semiconductor single crystal substrate for epitaxial growth of a Group III nitride semiconductor thereon. Thus, epitaxial growth of a Group III nitride semiconductor employs a substrate formed of a material other than Group III nitride semiconductor single crystal, such as sapphire (Al2O3) single crystal or silicon carbide (SiC) single crystal.
However, a large lattice mismatch exists between such a different-material substrate and a Group III nitride semiconductor crystal to be epitaxially grown on the substrate. For example, a 16% lattice mismatch exists between sapphire (Al2O3) and gallium nitride (GaN), whereas a 6% lattice mismatch exists between SiC and gallium nitride. In general, when such a large lattice mismatch exists between a substrate and a crystal to be grown thereon, it is difficult to epitaxially grow the crystal directly on the substrate, and the thus-grown crystal fails to exhibit good crystallinity. In view of the foregoing, in the case where a Group III nitride semiconductor crystal is to be epitaxially grown atop a sapphire single crystal substrate or an SiC single crystal substrate by means of metal organic chemical vapor deposition (MOCVD), there has generally been carried out a method disclosed in Japanese Patent No. 3026087 or Japanese Patent Application Laid-Open (kokai) No. 4-297023, in which a low-temperature buffer layer formed of aluminum nitride (AlN) or AlGaN is deposited onto a substrate, and a Group III nitride semiconductor crystal is epitaxially grown on the buffer layer at high temperature.
In addition to the aforementioned growth method employing a low-temperature buffer layer, there has been proposed a method (e.g., a method disclosed in Japanese Patent Application Laid-Open (kokai) No. 2003-243302) in which a Group III element source and a nitrogen source are fed onto a heated substrate such that the ratio of nitrogen to a Group III element becomes 1,000 or less, or merely a Group III element source (in the case where the nitrogen/Group III element ratio is zero) is fed onto the substrate, to thereby form a Group III nitride semiconductor; and subsequently a Group III nitride semiconductor single crystal is epitaxially grown by use of a Group III element source and a nitrogen source.
Meanwhile, a substrate having very low surface roughness is required for forming a Group III nitride semiconductor having a smooth surface and exhibiting excellent crystallinity. For example, Japanese Patent Application Laid-Open (kokai) No. 2002-255694 discloses a technique in which a buffer layer as described above is provided on a substrate having a surface roughness (Rms) of 0.1 nm or less and a surface roughness (Ra) of 0.06 nm or less, followed by growth of a Group III nitride semiconductor single crystal. However, when sapphire or silicon carbide, which has high hardness, is employed as a substrate for growing a Group III nitride semiconductor, a laborious step is required for attaining a very low surface roughness of the substrate, leading to an increase in production cost.
Japanese Patent Application Laid-Open (kokai) No. 2002-093726 discloses a technique for forming a Group III nitride semiconductor having reduced dislocation density and exhibiting excellent crystallinity, in which a substrate, on which a mask is provided, is subjected to etching to thereby form periodically arranged grooves on the surface of the substrate, and a Group III nitride semiconductor single crystal is grown on the substrate. However, provision of a mask on the substrate requires an intricate process, and increases the production cost.